iPhone Dev Team has quietly released the next revision of Redsn0w with iOS 5 user improvements and added the support for implementation of Custom Ramdisk commands using -r command line. You can read the full review about the newly released Redsn0w for iOS 5 here.

Redsn0w 0.9.9b4 Changelog as per developer,

Update #12: For those of you who experiment with your own custom ramdisks using the -r command-line option, version 0.9.9b4 adds auto-detection support for iOS5. This is needed because iOS4 and iOS5 treat the root partition differently (it’s encrypted in iOS5). redsn0w will now upload the correct iOS5 kernel by itself, but it’s up to your own launchd to determine if it needs to mount using the old or new partition scheme. Redsn0w now also accepts both native and img3-encapsulated versions of files you provide via the -r, -k, and -d command-line options (do redsn0w -h to show all the available options).

If you have already updated to iOS 5 GM then you need to go though the Tutorial posted here for getting tethered Jailbreak however in place you can use the latest Redsn0w 0.9.9b4 by downloading from the link below,

redsn0w_win_0.9.9b4 bug: crashes on my unlocked iPhone 3DS that I accidentally updated to the most recent OS from Apple two days ago (using the suggested by the program iPhone2,1_5.0_9A334_Restore.ipsw). No way to use the phone now. No carrier, no icons, just the grey background with a switch. And the “couldn’t activate” message